Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change,opened a meeting to discuss the panel’s latest report.
PARIS, Jan. 29 — Scientists from across the world gathered Monday to hammer out the final details of an authoritative report on climate change that is expected to project centuries of rising temperatures and sea levels unless there are curbs in emissions of carbon dioxide and other gases that trap heat in the atmosphere.
Scientists involved in writing or reviewing the report say it is nearly certain to conclude that there is at least a 90 percent chance that human-caused emissions are the main factor in warming since 1950. The report is the fourth since 1990 from the Intergovernmental Panel on Climate Change, which is overseen by the United Nations.
The report, several of the authors said, will describe a growing body of evidence that warming is likely to cause a profound transformation of the planet.
Three large sections of the report will be forthcoming during the year. The first will be a summary for policy makers and information on basic climate science, which is expected to be issued on Friday.
Among the findings in recent drafts:
¶The Arctic Ocean could largely be devoid of sea ice during summer later in the century.
¶Europe’s Mediterranean shores could become barely habitable in summers, while the Alps could shift from snowy winter destinations to summer havens from the heat.
¶Growing seasons in temperate regions will expand, while droughts are likely to ravage further the semiarid regions of Africa and southern Asia.
“Concerns about climate change and public awareness on the subject are at an all-time high,” the chairman of the panel, Rajendra Pachauri, told delegates on Monday.
But scientists involved in the effort warned that squabbling among teams and government representatives from more than 100 countries — over how to portray the probable amount of sea-level rise during the 21st century — could distract from the basic finding that a warming world will be one in which shrinking coastlines are the new normal for centuries to come.
Jerry Mahlman, an emeritus researcher at the National Center for Atmospheric Research in Boulder, Colo., who was a reviewer of the report’s single-spaced, 1,644-page summary of climate science, said most of the leaks to the news media so far were from people eager to find elements that were the most frightening or the most reassuring.
He added in an interview that such efforts distracted from the basic, undisputed findings, saying that those point to trends that are very disturbing.
He noted recent disclosures that there is still uncertainty about the pace at which seas will rise because of warming and the melting of terrestrial ice over the next 100 years. That span, he said, is just the start of a rise in sea levels that will almost certainly continue for 1,000 years or so.
Many economists and energy experts long ago abandoned any expectation that it would be possible to avoid a doubling of preindustrial carbon dioxide concentrations, given the growth of human populations, use of fossil fuels, particularly coal, and destruction of forests in the tropics.
The report is likely to highlight the hazardous consequences of that shift by finding that reaching twice the preindustrial concentration of carbon dioxide will probably warm climate between 3.5 and 8 degrees Fahrenheit and by highlighting that there is a small but significant risk that such a buildup can produce even more warming.
One major point of debate in early drafts of the report is the projection of a smaller rise in sea level than the last report as scientists relying on computer models and field observations struggle to find a consensus. Some scientists say that the figures used in the coming report are not recent enough because they leave out recent observations of instability in some ice sheets in Antarctica and Greenland.
Another possible point of contention during the four days of closed sessions in Paris this week may be assertions in early drafts of the report that the recent warming rate was blunted by particle pollution and volcanic eruptions.
Some scientists say the final report should reflect the assumption that the rate of warming in coming years is likely to be more pronounced than that of previous decades.
Achim Steiner, the executive director of the United Nations Environment Program, said the findings presented Friday should lead decision makers to accelerate efforts to slash carbon emissions and to help people in vulnerable parts of the world prepare for climate change.
“These findings should strengthen the resolve of governments to act now to reduce greenhouse gas emissions and put in place the medium- to longer-term strategies necessary to avert dangerous climate change,” Mr. Steiner said.
In a new report issued Monday, his agency said the most recent evidence from mountain glaciers showed that they were melting faster than before.
In the past year, international concern over what to do about global warming has grown along with concrete signs of climate change. Even so, political leaders are still groping for ways to tackle the phenomenon. Europe has adopted a program that caps the amount of emissions from industrial plants.
But the world’s largest emitter, the United States, still is debating whether to adopt a similar policy, while developing countries like China are resisting caps on the ground that the industrialized countries contributed about 75 percent of the current volume of greenhouse gases and should make the deepest cuts.
Many experts involved in the intergovernmental panel’s process said there was hope that with a prompt start on slowing emissions, the chances of seeing much greater warmth and widespread disruption of ecosystems and societies could be reduced.
Outside experts agreed.
“We basically have three choices: mitigation, adaptation and suffering,” said John Holdren, the president of the American Association for the Advancement of Science and an energy and climate expert at Harvard. “We’re going to do some of each. The question is what the mix is going to be. The more mitigation we do, the less adaptation will be required and the less suffering there will be.”
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